It has already been proposed to provide a process for the removal of particles from a flowing medium in a filter device operating by a repetition of three main working phases. The filter chamber is provided with closable inlet and outlet openings and can be formed with a support fabric for a filter layer, this fabric subdividing the chamber into two compartments. In each deposition phase, a filter-aid agent, i.e. a fibrous or particulate material, is entrained by a carrier medium through the filter cloth from an upstream compartment to a downstream compartment to deposit a layer of this filter aid in the form of a filter layer upon the support fabric. In each filtering phase of the sequence, the medium to be filtered is introduced into the upstream compartment and traverses the filter layer upon which its particles deposit. The purified filtrate is removed from the downstream compartment. The third or final main phase in the operation of such a system is a cleaning phase in which the contaminated filter layer is backwashed from the support fabric and removed from the upstream compartment.
A process of this type has been described in German Patent DT-PS 965 850. In this process, surface water is filtered to remove impurities. In this case, the backwashing uses a clarified or purified liquid, namely, the filtrate from a previous filtering stage.
This filtrate loss during the backwashing to release the deposited filter layer can be considered acceptable only if the filtered medium is not especially valuable or is not available only in limited quantities. This may be the case for water but is usually not the case for other liquids and hence this process has been found to be practical only for water filtration.
A similar process is described in AT-PS 316 583. In this process, backwashing is carried out with the filtrate in such manner that a mixing of the medium to be filtered with the filtrate during the cleaning phase is unavoidable. Since this process also deals only with the cleaning of water, the disadvantage is only of limited significance.
However, with the filtration of valuable liquids, which are only produced in small quantities, i.e. in a variety of processes in which losses of the filtrate are undesirable and a mixing of the washing medium and the medium to be filtered can occur, problems have been encountered with the prior-art systems.
This is especially the case in the production of wine and beer where filtration is necessary. In such cases, the filter elements must be removed, cleaned, and re-introduced into the processing line if contamination of the filtered product as a result of the backwashing is not to occur.
Such procedures are time-consuming and limit the size of the apparatus which can be utilized because of the need to manipulate the filter surfaces at frequent intervals. It is frequently necessary to provide a large number of filter surfaces to minimize the down time of the apparatus and increase the interval between two cleaning operations.
In filter systems for such comestible materials of relatively high value, the filter-aid agent is usually a high-quality expensive substance, such as kieselguhr (a silica) which is added to the medium during the filter process so that the filter process also involves a progressive increase in the size of the filter layer.
Both this type of filtering operation and the subsequent cleaning have been found to require extremely high operating pressures and the use of such high pressures necessitates the formation of a pressure-resisting filter device. Such filter units, capable of resisting the significantly elevated pressures required by the processes, are extremely expensive, create cleaning difficulties, and can only be operated at high energy cost.
Filter devices are also known for the recovery of solid particles. The residues of the medium to be filtered are removed as far as possible by pressing the filter cake formed on the filter layer. Such a system is described, for example, in Swiss patent CH-PS 399 422. The filter cake is then washed and dried, e.g. as described in German open application DT-OS 1 805 478.
For the pressing of the filter cake, movable walls, e.g. membranes, can be used and can be urged against one another by pressurization of compartments defined between these membranes and other walls of the filter chamber. The filtrate is usually intended to run off through grooves in the movable walls.
After the filter cake has been pressed, the filter device is usually opened and the filter cake removed. Obviously, in such case, it is of no significance that backwashing does not use the filtered medium.
The process there described is not satisfactory when the material removed on the filter layer is contaminated or is not readily handled. It will not be effective, moreover, when a coherent filter cake is not formed.
It is obvious from the foregoing that conventional filter systems, whether they use the backwashing technique or the filter cake removal technique, have disadvantages especially when the processes are intended for the removal of biological materials from a liquid.